water chemistry workshopaksinha

NETRA

5th Sep. 2010

ASHWINI K. SINHAAGM (NETRA)

[email protected]@gmail.com

NTPC Energy Technology Research Alliance (NETRA)NTPC LIMITED.E 3, Ecotech II, Udyog Vihar, Greater Noida 201308 (UP)FAX 0120-2350449

1

Introduction to Corrosion & Cooling Water Treatments

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Overview

• Corrosion Costs

• Corrosion Basics• • Basics of water treatment

• Problems of Cooling Water Treatment

• Online Monitoring of Cooling Water Systems

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Corrosion Activities at NETRA

Corrosion Analysis,

Monitoring & Control

Laboratory

3

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4

COST OF CORROSION

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5

COST OF CORROSION

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COST OF CORROSION

AS PER EPRI, USA, 2003 STUDIES:

Corrosion-related problems in fossil plant heat exchangers (condensers, Feed-water heaters, service water heat exchangers, lube oil coolers, etc.) have been estimated to cost approximately 360 million dollars per year in 1998 in the United States.

Corrosion products picked up in the heat exchangers can lead to increased deposition of copper and iron in the boiler, causing problems such as under-deposit corrosion, and to copper deposition in high pressure turbines, leading to power losses.

This aspect of the problem with condensers and heatexchangers was estimated to cost approximately $150 million per year

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Costs of Corrosion ProblemsAffecting Fossil Steam Plants

Corrosion Problem O&M Non- Fuel Related

Corrosion Cost

US $

Depreciation Corrosion Cost

US $

Total Corrosion Cost

US $

All Corrosion Problems in Fossil Steam Plants

3,43,50,00,000 1,14,20,00,000 4,57,70,00,000

Waterside/Steam side Corrosion of Boiler Tubes

91,60,00,000 22,84,00,000 1,14,44,00,000

Turbine CF & SCC 45,80,00,000 14,27,50,000 60,07,50,000

Oxide Particle erosion of Turbines

27,48,00,000 8,56,50,000 36,04,50,000

Heat Exchanger Corrosion 27,48,00,000 8,56,50,000 36,04,50,000

Fireside Corrosion of Water wall tubes

18,32,00,000 14,27,50,000 32,59,50,000

Generator clip to strand Corro 18,32,00,000 2,85,50,000 21,17,50,000

Copper deposition in turbines 9,16,00,000 5,71,00,000 14,87,00,000

Fireside Corrosion of SH & RH tubes

9,16,00,000 5,71,00,000 14,87,00,000

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Costs of Corrosion ProblemsAffecting Fossil Steam Plants

Corrosion Problem O&M Non- Fuel Related Corrosion

Cost US $

Depreciation Corrosion Cost

US $

Total Corrosion Cost

US $

Corrosion of FGD system 4,58,00,000 8,56,50,000 13,14,50,000

Liquid Slag Corrosion of Cyclone Boilers

9,16,00,000 2,85,50,000 12,01,50,000

Backend dew point corrosion 9,16,00,000 2,85,50,000 12,01,50,000

Generator Cooling water clogging & plugging

9,16,00,000 2,85,50,000 12,01,50,000

FAC of steam plant piping 9,16,00,000 2,85,50,000 12,01,50,000

Corrosion of service water, circulating water and other water systems

9,16,00,000 2,85,50,000 12,01,50,000

All other (Corrosion of structures, ash handling equipment, CHP, oil pipes & tanks, electrical equipment,

45,80,00,000 8,56,50,000 54,36,50,000

Total 3,43,50,00,000 1,14,20,00,000 4,57,70,00,000

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Corrosion is a natural process and is a result of the inherent tendency of metals to revert to their more stable compounds, usually oxides. Most metals are found in nature in the form of various chemical compounds called ores. In the refining process, energy is added to the ore, to produce the metal. It is this same energy that provides the driving force causing the metal to revert back to the more stable compound.

CORROSION

General Corrosion

Pitting Corrosion

Under deposit Corrosion

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R&D CENTRE

CORROSION

• “The deterioration of a material, usually a metal, that results from a reaction with its environment.”

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CORROSION IS A NATURAL PROCESS BY VIRTUE OFWHICH THE METALS TEND TO ACHIEVE THE

LEAST ENERGY STATE – I.E. COMBINED STATE

M M2+ + 2e- ANODIC REACTION

N 2- + 2e N

CATHODIC REACTIONMIC

Dezincification

WHAT IS CORROSION

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CORROSIONCORROSION

Fe Fe2+ + 2e-

Fe2+ + 2OH- Fe (OH)2

Fe (OH)2 + H2O + 1/2 O2 Fe (OH)3

Fe (OH)3 Fe2O3 . H2O + 2H2O

Anode

Cathode

2 H+ + 2e- H2

4H+ + O2 + 4e- 4 OH-

O2 + 2H2O + 4e- 4 OH-

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Corrosion in Power Plants

“Corrosion is a leading cause of loss of availability in both fossil and nuclear power plants. The financial impact of corrosion to the power industry is of the order of billions of dollars per year. Corrosion occurs within the steam cycle of nuclear and fossil power plants and on the fire-side of fossil fired power plants. Steam cycle related corrosion has been the most troublesome causing failures to major components such as boilers, steam generators, turbines, feedwater heaters, condensers and the piping throughout the steam plant. Failures on the fire-side of fossil plants include; boiler tubes, superheaters, scrubbers and other pollution control equipment. ”.

Schematic representation of locations ofCorrosion in Steam-Water cycle

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WHAT IS CORROSION

IRON OXIDE REFINING MILLING

STEEL CORROSION IRON OXIDE

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ELECTROCHEMICAL NATURE OF CORROSION

-600mV

-575mV

-550mV

Potential Differences on Steel Surface

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ELECTROCHEMICAL NATURE OF CORROSION

1) ANODE

2) CATHODE

3) ELECTROLYTE

4) ELECTRICAL CONNECTION

Anode-600mV

Cathode

-550mV

-575mV

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MECHANISM OF CORROSION

Corrosion Cell

Na+

Ca++

Cl-

SO4– -O2

OH

Fe++

H+Water

Anode Steel Cathode

Electrons

Fe+

Fe+ Fe+

Fe+

Fe(OH)2

Fe(OH)2 Fe++

OH–

O2

H+ H2

H+ H+

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PRACTICAL GALVANIC SERIES

Material Potential*

Pure Magnesium -1.75

Magnesium Alloy -1.60

Zinc -1.10

Aluminum Alloy -1.00

Cadmium -0.80

Mild Steel (New) -0.70

Mild Steel (Old) -0.50

Cast Iron -0.50

Stainless Steel -0.50 to + 0.10

Copper, Brass, Bronze -0.20

Titanium -0.20

Gold +0.20

Carbon, Graphite, Coke +0.30

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

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R&D CENTRE

CATHODIC PROTECTION - A TECHNIQUE TO PREVENT

CORROSION OF POWER PLANT COMPONENTS.

DIAGRAM pH - STEEL POTENTIAL IN WATER AT 25° C (POURBAIX)

Hyd

ro

gen

ele

ctro

de

Cu

/Cu

SO

4ele

ctro

de

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C.R (mpy) 22.3 x WD x A x T

MPY = Mils Per Year

W = Weight Loss (mg)

D = Specific Gravity (gm / cm2)

A = Exposed Area (inch2)

T = Time (Days)

1 MM = 40 Mils

1 MIL = 25 Microns

CORROSION RATE

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Metals to be considered

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FORMS OF CORROSION

GENERAL LOCALIZED GALVANIC

ENVIRONMENTAL CRACKING

VELOCITY EFFECT INTERGRANULAR

DEALLOYING FRETTINGHIGH-

TEMPERATURE

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corrosion or degradation of material exposed to the air and its pollutants rather than immersed in a liquid

corrosion that occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte

caused by an externally induced electrical current

corrosion of metals generally over the entire exposed surface in aqueous environments.

corrosion of metals due to molten or fused salts

types of corrosion found in liquid liquid metals metal / containment / component combinations

corrosion by direct reaction of exposed metals to oxidizing agents at elevated temperatures

General / Uniform Corrosion :

Corrosive attack dominated by uniform thinning due to even regular loss of metal from the corrosion surface

Atmospheric

Galvanic

Stray-current

General biological

Molten salt

Liquid metals

High-temperature

FORMS OF CORROSION

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Removal of surface material by the action of numerous individual impacts of solid or liquid particles

Combined wear and corrosion between contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations

Occurs on a metal surface in contact with a liquid, pressure differentials generate gas or vapor bubbles which upon encountering high pressure zones, collapse and cause explosive shocks to the surface

Occurs in metals as a result of the combined action of a cyclic stress and a corrosive environment

Mechanically assisted degradation :

Form of attack where velocity, abrasion, hydrodynamics etc. play a major role

Erosion

Fretting

Cavitation & Water drop impingement

Fatigue

FORMS OF CORROSION

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Occurs on metallic surfaces coated with thin organic film, typically. 1 mm thick, characterized by the appearance of fine filaments in semi-random directions from one or more sources

Corrosion in narrow openings or spaces in metal to metal or non-metal to metal component sites

Extremely localized corrosion marked by the development of pits

Cases where biological organisms are the sole cause or an accelerating factor in the localized corrosion

Occurs when the corrosion rate of the train boundary areas of an alloy exceeds that of the grain interiors

A form of corrosion characterized by the preferential removal of one constituent of an alloy leaving behind an altered residual structure

LocalizedCorrosion : All or most of the metal loss occursat discrete areas

Metallurgi-cally influenced Corrosion :Form of attackwhere velocity, abrasion, hydrodynamics etc. play a major role

Filiform

Crevice

Pitting

Localizedmicrobiological

Intergranular

Dealloying

FORMS OF CORROSION

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Service failures in engineering materials that occur by slow environmentally induced crack propagation

Results from the combined action of hydrogen and residual or tensile stress

Brittle failure of a normally ductile metal when coated with a thin film of a liquid metal and subsequently stressed in tension

Occurs below the melting point of the solid in certain liquid metal embrittlement couples

Environmentally inducedcracking : Forms of cracking that are produced in the presence of stress

Stress cracking

Hydrogen damage

Liquid metal embrittlement

Solid metal embrittlement

FORMS OF CORROSION

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ALLOY ENVIRONMENT

Aluminium Base

Magnesium Base

Copper Base

Carbon Steel

Martensitic & Precipitation Hardening Stainless Steels

Austenitic Stainless Steels

Nickel Base

Titanium

• Air• Seawater• Salt & Chemical Combinations• Nitric Acid• Caustic• HF Solution• Salts• Coastal Atmospheres• Primarily Ammonia & Ammonium Hydroxide• Amines• Mercury• Caustic• Anhydrous Ammonia• Nitrate Solutions• Seawater• Chlorides• H2S Solutions• Chlorides Inorganic & Organic• Sulfurous & Polythionic Acids• Caustic Solutions• Caustic Above 600F (315 C)• Fused Caustic• Hydrofluoric Acid• Seawater• Salt Atmospheres• Fused Salt

ENVIRONMENT/ALLOY SYSTEMS SUBJECT TO STRESS CORROSION CRACKING

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High-temperature corrosion

spolled oxide scale

high-temperature oxygen

metal

HERE’S HOW YOU CAN SPOT THE MANY COMMON VARIETIES OF CORROSION

It can show up in a host of ways and forms. And many of the most common types of corrosion

conditions overlap each other

Concentration cells

high- oxygen concentration

corrosion currentlow oxygen concentration

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Uniform attack

electrolyte

corrosion product

onode metalPitting corrosion

pit (onode)

corrosion currentmetal

corrosion product

(cathode)

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Intergranular corrosion

intercrystalline crack




Stress corrosion cracking

stress-corrosion cracks

load

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Corrosion fatigue

cyclic loading

fatigue crocks

Selective attack

plug-type dezincification

layer-type dezincification


It can show up in a host of ways and forms. And many of the most common types of corrosion conditions

overlap each other

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Impingement attack

corrosion filmflow

impinging stream

Fretting corrosion

fretting at fight fits subject to vibration


It can show up in a host of ways and forms. And many of the most common types of corrosion conditions overlap

each other

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• Reduced life of components

• Reduced efficiency of equipment

• Reduced availability of plant equipment

• Reduced reliability of equipment & structures

• Endanger to life of people around

• Enhanced maintenance

• Contaminations in process fluids

• Secondary failures in other associated equipment

• Higher costs of generation

Effects of Corrosion in Power Plants

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TYPE OF CORROSION AFFECTED COMPONENT

GENERAL CORROSION STRUCTURES

UNDER DEPOSIT CORROSION WATER WALL TUBES, CONDENSERTUBES, L.P.HEATERS

PITTING STAINLESS STEEL TUBES, TURBINE BLADES

STRESS CORROSION CRACKING SH TUBES, L.P.TURBINE BLADESEND RETAINING RINGS, COND. TU

INTERGRANULAR CRACKING SUPER HEATERS, L.P.HEATERS

CREVICE ATTACK BOILER DRUM, CONDENSER TUBES

GALVANIC CORROSION WATER BOXES, PUMPS, PIPES

HYDROGEN EMBRITTLEMENT WATER WALL TUBES, STEAMTURBINES

CAUSTIC GOUGING WATER WALL TUBES

EROSION CORROSION FEED PIPELINES, ASH PIPELINES

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TYPE OF CORROSION AFFECTED COMPONENT

ATMOSPHERIC CORROSION STRUCTURES

AQUEOUS CORROSION WATER BOXES, CW DUCTS, SCREENS, CLARIFIER BRIDGES

SOIL SIDE CORROSION UG FIRE WATER & AUXILIARYCOOLING WATER PIPELINES

MICROBIOLOGICALLY INDUCED UG PIPES, CW SYSTEMS,CORROSION FIRE WATER PIES (INTERNALS)

DEZINCIFICATION CONDENSER TUBES, L.P.HEATERTUBES

HIGH TEMP. OXIDATION GAS TURBINE BLADES

ACID DEW POINT CORROSION WHRBs, AIR PRE HEATERSSTACK LINERS, DUCTS

CAVITATION PUMPS, VALVES

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Corrosion in Boiler tubes The major reasons of boiler

tube leakage have been identified:

• Caustic Gauging• Hydrogen Embrittlement• Over heating due to high

internal oxides• Localized Corrosion under the

oxides• Loss of efficiency due to heavy

build up of oxides• Pitting attack

Hydrogen embrittlementCopper deposition

Caustic gauging Local acidic attackRecommendations

• Post operational chemical cleaning of boilers at regular intervals or when ever the internal oxide thickness increases beyond 50 microns

• Corrections in operating water chemistry

• Regular monitoring of oxide characteristics

• Unit shut down in case of condenser tube leakage

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Corrosion of low pressure and high pressure heaters Low pressure tubes made of

admiralty brass - Brass• Circumfrencial cracks, signs of

erosion-Corrosion, stress corrosion cracking & De-Zincification.

High pressure tubes made of admiralty brass - carbon steel

• tube thinning, perforations, pits & brownish deposit – Erosion Corrosion

Recommendations

• LP Heaters – tube material replacement by CU-Ni or SS

• HP heaters – tube material replacement by SS

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Corrosion in Structures of Coal handling Plant

• Support structures of coal conveyor belt - incorrect design of the support member & improper coating

Recommendations • Modified design of base of the

support member so as to ensure complete removal for coal slurry from the joint of concrete & member

• Replacement of damaged members by new members

• Application of coal tar based epoxy coating after proper surface preparation through qualified applicators

• Avoidance of accumulation of coal slurry at the base of the members

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Corrosion in Turbine

• LP Turbine lacing wire failures - stress corrosion cracking.

• Hot Corrosion of Gas Turbine - chromium depletion under hot conditions

• LP Turbine Blade Failure - stress corrosion cracking

Recommendations

• LP Turbine lacing wire failures – Replacement of lacing wires after stress relieving

• LP Turbine Blade Failure – Prevention of ingress of sodium

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Corrosion in HRSGs

• Acid Dew Point Corrosion in liquid fuel firing

Recommendations

• Alkaline water wash .

• Washing/treatment with Magnesium hydroxide can reduce the impact of acid dew point corrosion.

• Anticorrosive coatings for structures

• Control of exit gas temperature on the skin of boiler tubes

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Corrosion in Fuel Tanks & Misc Structures

• Corrosion in Naptha tanks • RCC corrosion of cooling

towers• Corrosion of ESP electrodes• Corrosion of ash handling

systems• Atmospheric corrosion• Corrosion of pumps• Corrosion of DM plant

structures• Corrosion of structures of

clarifiers Recommendations

• Cathodic protection • Anticorrosive coatings• Change of electrode material

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Corrosion in Coal Wagons • surfaces of the coal wagons of

the MGR system were found to be severely corroded and had developed perforations - Continuous contact with high sulphur moist coal was identified to be the main reason of failure

Recommendations • Change the material of

construction to Corten steel • Anticorrosive coatings

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TYPICAL COOLING WATER QUALITY AT A STATION

S.No. SamplesParameters

RIVER WATER

Circu. Water

Clarified water

Raw Water

CW Sump

1. pH 8.57 7.6 8.13 8.48

2. Conductivity 415 248 230 410

3. P-alkalinity ppm 5.0 Nil Nil 5.0

4. M-alkalinity ppm 118 67 78 115

5. Ca-hardness ppm 106 60 64 100

6. Mg-hardness ppm 52 28 31 50

7. Chlorides ppm 27 12 05 25

8. Sulphates ppm 66 37 28 58

9. KMnO4 1.6 1.0 2.4 1.0

10. Silica ppm 7.7 5.9 6.7 7.0

11. Turbidity NTU 10 10 75 15

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TYPICAL COOLING WATER QUALITY AT A STATION

S.No MONTH Cond.mho/cm

pH Hardness (ppm) Cl- H2S M Alk. (ppm)

NH3 Oil

TH CaH MgH

1. Apr.05

Max 62280 8.4 6350 1100 5250 19896 0.6 190 TR TR

Min 41220 8.0 5350 850 4500 14889 NT 175 NT NT

2. May05

Max 55660 8.5 7000 1250 5750 18257 NT 190 NT NT

Min 42500 8.5 5860 1010 4850 15812 NT 180 0.1 NT

3. June05

Max 59760 8.3 5380 1150 4300 17725 0.3 200 TR NT

Min 43720 8.2 4800 1050 3750 16378 NT 190 NT NT

4. July05

Max 59750 8.4 5500 1250 4600 16474 0.2 275 TR NT

Min 35950 8.2 5050 900 3800 14322 NT 210 NT NT

5. Aug.05

Max 57500 8.3 4550 100 3550 16130 0.7 250 TR NT

Min 39000 8.0 4300 850 3450 14889 NT 180 NT NT

SEAWATER

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Impact of Condenser tube leakageon ingress in boiler water

Analysis of Circ. Water

(RIVER)

Amt of diff. constituents in 1% of tube leak, g/hr

Condensate flow, m3/hr

Increase in 1 hour (ppb) Increase in 24 hr (ppb)

Param. Value ST 1 ST 2 ST 1 ST 2 ST 1 ST 2 ST 1 ST 2

pH 7.5 700 1500

Cond 115

TH 41 1.417 2.323 2.0 1.5 48.6 37.2

CaH 29 1.003 1.643 1.4 1.1 34.4 26.3

MgH 12 0.415 0.680 0.6 0.5 14.2 10.9

Cl 5 0173 0.283 0.2 0.2 5.9 4.5

SO4 12 0415 0.680 0.6 0.5 14.2 10.9

P-alk 0 0 0 0 0 0 0

M-alk 36 1.245 2.040 1.8 1.4 42.7 32.6

TDS 0 0 0 0 0 0

Na/K 12 0.415 0.680 0.6 0.5 14.2 10.9

SiO2 8.3 0.287 0.470 0.4 0.3 9.8 7.5

Reservoir Water

CW Flow m3/hr

No. of tubes Tubes in 1 pass

Water flow/ tube m3/hr

1% flow/ tube L/hr

ST 1 27000 15620 7810 3.5 34.6

ST 2 70000 24710 12355 5.7 56.7

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Sea water CW Flow m3/hr



1% flow/ tube L/hr

24000 13600 6800 3.5 35.3

Analysis of Circ. Water

(SEAWATER)

Amt of diff. constituents in 1% of tube leak, g/hr


Increase in 1 hour (ppb)

Increase in 24 hr (ppb)

Param. High Low High Low High Low High Low

pH 8.4 8

Cond 62280 270

TH 6350 70 224.118 2.471 700 320.2 3.5 7684 84.7

CaH 1100 50 38.824 1.765 55.5 2.5 1331.1 60.5

MgH 5250 20 185.294 0.706 264.7 1.0 6352.9 24.2

Cl 19896 46 702.212 1.624 1003.2 2.3 24075.8 55.7

SO4 0 0 0 0 0 0

P-alk 0 0 0 0 0 0

M-alk 190 50 6.706 1.765 9.6 2.5 229.9 60.5

TDS 29657 129 1046.723 4.538 1495.3 6.5 35887.6 155.6

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Canal Water & Borewell water

CW Flow m3/hr



1% flow/ tube L/hr

22500 15330 7665 2.9 29.4

Analysis of Circ. water Amt of diff. constituents in 1% of tube leak, g/hr


Increase in 1 hour (ppb)

Increase in 24 hr (ppb)

Param. Canal Bore wee

Canal Bore well

Canal Bore well

Canal Bore well

pH 8.9 9.07 700

Cond 976 2279

TH 12 23 0.352 0.675 0.5 1.0 12.1 23.1

CaH 8 14 0.235 0.411 0.3 0.6 8.1 14.1

MgH 4 9 0.117 0.264 0.2 0.4 4.0 9.1

Cl 107 225 3.141 6.605 4.5 9.4 107.7 226.4

SO4 147 85.5 4.315 2.510 6.2 3.6 147.9 86.0

P-alk 29 187.9 0.851 5.516 1.2 7.9 29.2 189.1

M-alk 240 700.4 7.045 20.560 10.1 29.4 241.5 704.9

TDS 585.6 1367.4 17.190 40.139 24.6 57.3 589.4 1376.2

Na/K 482 987.9 14.149 28.999 20.2 41.4 485.1 994.3

SiO2 27.32 31.5 0.802 0.925 1.1 1.3 27.5 31.7

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Issues involved in Management of CW systems at Fossil Power Plants

1. Identification of proper constant source of water at a power station

2. Identifying application areas

3. Characterization of water quality

4. Selection of pretreatment suiting specific application

5. Assessment of water quality and quantity

6. Identification of Application areas for water

7. Assessment of requirements for suitable chemical treatment to controlof scaling, fouling, corrosion, & biofouling etc

8. Environmental Considerations

9. Safety Considerations

10.Assessment of recycling possibilities of waters and waste waters

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Process Flow Schematic for Wet Recirculating Cooling Water System

1 GPM = 0.2271 M3/hour

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1 GPM = 0.2271 M3/hour

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Water Requirements

Water Requirements for a 500 MW Unit (Ref. Flow schematic)

S.No. Description

Flow rate

GPM M3/Hr

1Boiler Feedwater 7645 1736.18

2Circulating Water 187600 42603.96

3Evaporation & Drift 6415 1456.847

4Make up 9537 2165.853

5Blowdown water 3161 717.8631

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Power Plant Water ChemistryInfluenced Damages

Chemistry influenced component damage in fossil plant units is widespread and includes the following mechanisms.

• Condenser tubes: steamside initiated stress corrosion cracking, pitting from the steam side, condensate or ammonia grooving.

• Condenser structure: flow-accelerated corrosion of steam side shell, supports, headers and piping.

• Deaerators: flow-accelerated corrosion, pitting, corrosion fatigue, and stress corrosion cracking.

• Feedwater heaters and associated piping: general corrosion and pitting, corrosion fatigue, flow-accelerated corrosion, stress corrosion cracking, and deposits.

• Economizer tubes: pitting, flow-accelerated corrosion and corrosion fatigue.

• Boiler tubes: hydrogen damage, acid phosphate corrosion, caustic gouging, corrosion fatigue, pitting, and deposit induced overheating.

• Superheaters and reheaters: pitting, stress corrosion cracking, and corrosion fatigue.

• Turbines: corrosion fatigue, erosion and corrosion, stress corrosion cracking, crevice corrosion, pitting, and deposits (reducing efficiency and capacity).

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Water Requirements at Power Plants“Water, once considered a nearly inexhaustible resource, is increasingly limited, and water requirements for electricity production must compete with other demands, such as agriculture and sanitation”.

Water is the life & blood of fossil power plants.

A 200 MW once-through coal fired unit requires around 40000 m3/hour of water for various applications such as boiler water, service water, condenser cooling water, fire water, equipment cooling water, etc.

Bulk of this water is used for condenser cooling (around 30000 m3/hour).

For a re-circulating type of cooling water system (CW System) operating at 2.5 COC

approximately requires 5.5 m3/MW of water

For 75000 MW capacity envisaged by NTPC the water requirement will be in

excess of 41,25,000 m3/hour (if all units were being operated at 2.5 COC) 53

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Water Requirements at Power Plants

Cooling water requirements for each type of plant based on NETL data are:

• Coal, once-through, subcritical, wet FGD - 0.52 litres/kWh

• Coal, once-through, supercritical, wet FGD - 0.47 litres/kWh

• Nuclear, once-through, subcritical - 0.52 litres/kWh

• Coal, recirculating, subcritical, wet FGD - 1.75 litres/kWh

• Coal, recirculating, supercritical, wet FGD - 1.96 litres/kWh

• Nuclear, recirculating, subcritical - 2.36 litres/kWh

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Water Consumption

U.S. Freshwater Withdrawal (2000 )

U.S. Freshwater Consumption (1995)

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Water Consumption

Average Cooling System Water Use and Consumption

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Evaporation+drift

10500 M3/Day

COOLING TOWERCONDENSER

648000 M3/Day

AUX

Gur

gaon

Can

al

RESERVOIR

MakeUp12500 M3/Day

13000 M3/Day

BLOW DOWN2000 M3/Day

DMP Effluent

5M3/day

Boiler Blow Down

10 M3/day

Raw Water Pumps for CW

CW Pumps

COC = 5

So

fte

ne

r

Service Water

5M3/day

DM PlantClW

WATER

Clarified water

CMB

Clarifier

Raw Water Pumps for

DM

2 PUMPS

One Standby

Dis

cha

rge

to

Ag

ra C

an

al

60 M3/Hr

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SCHEMATIC DIAGRAM OF RAW WATER/COOLING WATER SYSTEM COAL STATION

Softening Plant

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Important Properties of Water

1. Conductivity

2. Hardness

3. Alkalinity

4. pH

5. Silica

6. Other impurities

-- Iron, Manganese,

Chlorides, Phosphate, etc.

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CHARACTERISTICS OF WATER & SOURCES OFIMPURITIES

PHYSICAL PROPERTIES:• DENSITY - 1.0 AT 4 oC

• Sp. Heat - 4.18 Kj/kg oC AT 0 oC

• Latent Heat - Fusion 79 kcal/KG

• Latent Heat - Vaporization 539 kcal/kg

• Viscosity 1.797 mPa.s at 0 oC

• Surface Tension 75.60 dynes/cm

• Osmotic Pressure

= CRT

: Osmotic Pressure in Pa

C : Diff. In Conc. In mol/m3

R : Constant of Ideal gases 8.314 J/mol/K

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MAKE - UP WATER TREATMENTS

• CLARIFICATION - ALUM, PEs, PAC

• FILTERATION

• ULTRAFILTERATION

• NANOFILTERATION

• REVERSE OSMOSIS

• ACTIVATED CHARCOAL

• SOFTENING - LIME, ION EXCHANGE

• ACID TREATMENT

• INHIBITORS

• OXYGEN REMOVAL

• BIOCIDE ADDITION

NETRA

EFFECT OF CLARIFICATION PROCESS ONCORROSION BEHAVIOUR OF COMPONENTS

• ALUM REDUCES pH, INCREASES SULPHATES

• INCREASED CORROSION

• PAC - LESS pH REDUCTION, LESS CORROSION

• POSSIBLE LESS COST

NETRA

Priorities for Corrosion Research & Development Priorities for Corrosion Research & Development

Reducing Condenser Tube Leakage:

• Development of sensors for composite online & centralized monitoring of scaling, fouling, biofouling & corrosion behaviour of cooling waters

• Corrosion audit of CW Systems to get the impact on the system & develop site specific remedial measure

• Corrosion Control measures to be taken at the design stage based on corrosion assessment studies

• Development of online condenser tube seepage detection based on conductivity measurements

• Development of acoustics method for condenser tube leakage

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Reducing Condenser Tube Leakage:

• Cleaning of fill packs of cooling towers by Off-line & online cleaning methods.

• Development of in-house site specific chemical treatments

• Development of site specific treatment technology for contaminated water for use in cooling water systems

• Development of alternate biofouling control technology

• Recommendations for anticorrosive coatings for CW systems

• Design of cathodic protection systems

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Reducing Turbine Deposit & Corrosion:

• Modeling & Simulation of steam flow path for on-line prediction of steam turbine condition

• Development of analytical procedure and control technique for colloidal silica

Reducing Acid Dew Point Corrosion of HRSGs/Boiler systems:

• Development of corrosion inhibitors for preventing acid dew point corrosion

• Modeling and simulation of ID Fan loading through extraction of moisture from flue gases

• Pilot scale studies on extraction of moisture, acidic gases and waste heat from flue gases

NETRA


Reducing Structural Corrosion:

• Development of Corrosion audit procedures for plant structures

• Development of metal spray coatings for corrosion protection of plant structures

Reducing Corrosion of Auxiliaries:

• Development of anticorrosive coating systems for different systems

• Design of cathodic protection systems for underground & submerged structures/pipelines waste heat from flue gases

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THE COOLING PROCESS

70

The purpose of cooling systems is to transfer heat from one substance to another

The substance that gives up its heat is “cooled”

The substance that receives the heat is the “coolant”

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Cooling System Problems

CORROSION

MICROBIO

FOU

LINGS

CA

LE

Left unchecked these problems cause

Loss of heat transfer

Reduced equipment life

Equipment failures Lost production Lost profits Increased

maintenance costs Plant shutdown

71

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TYPES OF COOLING WATERS

•ONCE THROUGH

• OPEN RECIRCULATING

• CLOSED RECIRCULATING

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• Water is relatively inexpensive and is an excellent transporter of heat. It also is an excellent polar solvent and eventually will dissolve just about all known materials. For this reason, the chemistry of all cooling water treatment programs must begin by addressing corrosion.

• Increasing water usage have combined to decrease the availability and increase the cost of the good quality, low-hardness water preferred for cooling tower makeup use. At the same time, stricter environmental restrictions on effluent discharge have resulted in increased fees for disposal of cooling tower blowdown to the sewers. The addition of these concerns to the existing requirements for control of scale, corrosion and biological fouling has increased the difficulty and costs associated with operating a cooling water management program.

• The water requirement for a coal-based power plant is about 0.005-0.18 m3/kwh while that for a natural gas plant is about 0.003 m3/kwh. This implies that for a 200 MW once through type coal based unit the water requirement is of the order of 40000 m3/hr. Major portion of this is utilized in condenser cooling (~ 30000 m3/hour).

Problems of Cooling Water Systems

73

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• Other usages of water include boiler feed water, ash slurry preparation water, coal dust suppressing water, service water, auxiliary cooling water, fire water, etc. Most of Indian power plants utilize fresh water (river, lake, reservoir, irrigation canal, borewell, etc) for the purpose. A few power plants use sea water for such applications.

• In spite of all these concerns, cooling water is a commonly neglected area often responsible for substantial problems due to downtime, equipment damage, loss of process control, high water use, environmental violations, safety hazards and increased energy use. Neglect of cooling water generally is because of two major reasons. First, the user often does not appreciate that cooling water is a vital part of the facility operation or production process; and second, misinformation, fraudulent products and marketing hype are common when cooling water treatment is the issue.

Problems of Cooling Water Systems

74

NETRA

HARD CRYSTALINEDEPOSIT OF SALTS OFCa & Mg, SiO2, PO4

PROBLEMS

SCALINGCORROSION

FOULING

GENERALFOULING

MICROBIOLOGICALFOULING

ELECTROCHEMICALREACTIONSINVOLVING O2

Porous Depositsdirt,Silts,Sandcorrosionproducts

Algaefungibacteria

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Mineral Scale

• Cooling Water contains many different minerals -- normally these minerals are dissolved in the water

• Under certain conditions minerals can come out of solution and form into hard, dense crystals called SCALE

NETRA

Scaled Heat Exchanger Tubes

Mineral Scale

Common Scales• Calcium Carbonate• Magnesium Silicate• Calcium Phosphate• Calcium Sulfate• Iron Oxide• Iron Phosphate• Others...

CaPO4

CaCO3

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Mineral Scale

The Following Factors AffectScale Formation...

Mineral ConcentrationWater TemperatureWater pHSuspended SolidsWater Flow Velocity

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Mineral Scale

• Scale usually forms in hot areas of cooling systems

• Reduces heat transfer efficiency

• Mechanical/Chemical cleaning

• Under deposit corrosion (pitting)

• Plant shutdown

• Equipment replacement

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Preventing Mineral Scale

• Limit concentration of scale forming minerals: Blowdown, clarify/filter MU

• Feed acid to reduce pH & alkalinity: Reduces scaling -- increases corrosion

• Mechanical design changes: Increase HX water velocity, backflush, air rumble

• Apply chemical scale inhibitors

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Mineral Scale

Three Classifications Of ScaleInhibiting Chemicals Are…

• Crystal Modifiers– Prevent scale from “laying down”

• Sequestrants– Prevent scale from agglomerating

• Dispersants– Affect mineral charge so that scale

formers repel each other

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CORROSION

Corrosion is the mechanism by which metals are reverted back totheir natural “oxidized” state

NETRA

Battery Analogy• Anode• Cathode• Electrical Circuit• Metal lost at anode

Corrosion

e-

Electrolyte

An

od

e

Cat

ho

de

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Simplified Corrosion Cell

Fe 2+

CATHODE

ANODE

O2

OH-

e-

STEP 1

STEP 2

STEP 3

STEP 4

Water withDissolved Minerals

Base Metal

O2

e-e- e-

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Four Step Corrosion Model

• Step 1: At the anode, pure iron begins to break down in contact with the cooling water. This step leaves behind electrons.

• Step 2: Electrons travel through the metal to the cathode.

• Step 3: At the cathode, a chemical reaction occurs between the electrons and oxygen carried by the cooling water. This reaction forms hydroxide.

• Step 4: Dissolved minerals in the cooling water complete the electrochemical circuit back to the anode.

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Factors Influencing Corrosion

• pH

• Temperature

• Dissolved Solids

• System Deposits

• Water Velocity

• Microbiological Growth

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100

10

05 6 7 8 9 10

Co

rro

sio

n R

ate,

Rel

ativ

e U

nit

s

pH

Corrosion Vs. pH

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Corrosion Vs. Temperature

Corrosion Rate

Tem

per

atu

re

In general, for every 18°F in water temperature, chemical reaction rates double.

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Other Causes of Corrosion

System Deposits• Anodic pitting sites develop under

deposits

Water Velocity• Too low = deposits• Too high = Erosion

Microbiological Growth• Deposits; Produce corrosive by-products

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Types of Corrosion

All cooling system metallurgy experiences some degree of corrosion. The objective is to

control the corrosion well enough to maximize the life expectancy of the system...

1. General Corrosion

2. Localized Pitting Corrosion

3. Galvanic Corrosion

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Base Metal

General Etch Uniform Attack

Water

Original

Thickness

General Corrosion

• Preferred situation

• Take a small amount of metal evenly throughout the system

• Anode very large

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Base Metal

Localized Pitting Attack

WaterOriginal

Thickness

Pitting Corrosion

• Metal removed at same rate but from a much smaller area

• Anode very small• Often occurs under

deposits or weak points

• Leads to rapid metal failure

NETRA

Galvanic CorrosionActive End

Passive End

MagnesiumGalvanized SteelMild SteelCast Iron18-8 Stainless Steel Type 304 (Active)18-12-3 Stainless Type 316 (Active)Lead TinMuntz SteelNickel (Active)76-Ni-16 Cr-7 Fe Alloy (Active)BrassCopper70:30 Cupro Nickel67-Ni-33 Cu Alloy (Monel)Titanium18-8 Stainless Steel Typ 304 (Passive)18-12-3 Stainless Steel Type 316 (Passive)GraphiteGoldPlatinum

• Occurs when two different metals are in the same system

• More reactive metal will corrode in presence of less reactive metal

• Potential for galvanic corrosion increases with increasing distance on chart

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Affects of Corrosion• Destroys cooling system metal• Corrosion product deposits in heat exchangers• Heat transfer efficiency is reduced by deposits• Leaks in equipment develop• Process side and water side contamination

occurs• Water usage increases• Maintenance and cleaning frequency increases• Equipment must be repaired and/or repaired• Unscheduled shutdown of plant

NETRA

Methods To Control Corrosion

• Use corrosion resistant alloys: $• Adjust (increase) system pH:

Scale• Apply protective coatings:

Integrity• Use “sacrificial anodes”: Zn/Mg• Apply chemical corrosion

inhibitors

NETRA

Fouling

FOULING is the accumulation of solid material, other than scale, in a way that hampers the operation of equipment or

contributes to its deterioration

NETRA

Common FoulantsSuspended Solids

• Silt, Sand, Mud and Iron

• Dirt & Dust

• Process contaminants, e.g. Oils

• Corrosion Products

• Microbio growth

• Carryover (clarifier/lime softener)

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• Water Characteristics

• Water Temperature• Water Flow

Velocity• Microbio Growth• Corrosion• Process Leaks

Factors Influencing Fouling

NETRA

Affects of Fouling

• Foulants form deposits in hot and/or low flow areas of cooling systems

• Shell-side heat exchangers are the most vulnerable to fouling

• Deposits ideal for localized pitting corrosion

• Corrosive bacteria thrive under deposits

• Metal failure results

NETRA

Economic Impact of Fouling

• Decreased plant efficiency• Reduction in productivity• Production schedule delays• Increased downtime for maintenance• Cost of equipment repair or

replacement• Reduced effectiveness of

chemical inhibitors

NETRA

Fouling

Three Levels Of Attack Can Be Employed To Address The

Effects Of Fouling...

1. Prevention

2. Reduction

3. Ongoing Control

NETRA

Preventing Fouling

Prevention• Good control of makeup quality• Good control of corrosion, scale, & microbio

Reduction• Increase blowdown• Sidestream filter

Ongoing Control• Backflushing, Air rumbling, Clean tower basin• Chemical treatment

NETRA

Preventing Fouling

Prevention• High Efficiency Multimedia Filters

– Capable of 80% removal of 0.5 micron

– Typical multimedia depth filters capable of 80% removal only down to 10 micron

– Most (greater than 90%) of particles found in a cooling tower are less than 10 micron

• Do not overlook sidestream filtration and choose wisely!

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FoulingChemical Treatment

• Charge Reinforcers– Anionic polymers increase strength

of charge already present on suspended solids

– Keep particles small enough so they do not settle out

• Wetting Agents– Surfactants– Penetrate existing deposits– Wash away from metal surfaces

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Microbiological Growth

• Water treatment is about managing three fouling processes...

CorrosionScaleMicrobio

The microbial fouling process is...

• The most complex• The least understood• The hardest to

measure and monitor• Controlled using the

least desirable, most expensive, & potentially hazardous products

NETRA


Three Kinds Of Troublesome Microorganisms In Cooling

Water...

1. Bacteria

2. Algae

3. Fungi/Mold/Yeast

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Bacteria

Types of Bacteria

1. Slime Forming

2. Anaerobic Corrosive

3. Iron Depositing

4. Nitrifying

5. Denitrifying

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Bacteria

Slime Formers

Iron DepositingAnaerobic

Typical Rods

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Bacteria• Produce acidic waste that lowers pH and

causes corrosion• Produce large volumes of iron deposits that

foul• Produce acids from ammonia that increase

corrosion & lower pH• Form sticky slime masses that foul & cause

reduced heat transfer

NETRA

Two Classifications of Bacteria

Planktonic:• Free-floating bacteria in bulk water

Sessile:• Bacteria attached to surfaces• Over 95% of bacteria in a cooling

system are sessile and live in BIOFILMS

NETRA

Biofilms

• Contribute to all cooling water problems

• Underdeposit corrosion

• Trap silt & debris which foul heat exchangers and tower fill

• Provide nucleation sites for scale formation Biofilm Formation

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Formation of biofilm

112

NETRA

FLOW

ThermalFoulant Conductivity

CaCO3 1.3-1.7CaSO4 1.3CaPO4 1.5MgPO4 1.3Fe Oxide 1.7Biofilm 0.4

P P

Common biofilms are 4 times more insulating than CaCO3 scale!

Biofilms

• More insulating than most common scales

• Reduce heat transfer efficiency

• Increase dP across heat exchangers & reduce flow

• Health risks (legionella)

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Algae

• Require sunlight to grow

• Found on tower decks & exposed areas

• Form “algae mats”

• Plug distribution holes on tower decks

• Plug screens/foul equipment

• Consume oxidants

• Provide food for other organisms

NETRA

Fungi

• Use carbon in wood fibers for food

• Destroy tower lumber by either surface or internal rotting (deep rot)

• Loss of structural integrity of tower

NETRA

Factors Affecting Growth of Microorganisms

• Microorganism Sources: Air or Makeup water

• Cooling systems provide the ideal environment for microbiological growth – Nutrients: Ammonia, oil, organic

contaminants– Temperature: 70-140°F acceptable– pH: 6.0 - 9.0 ideal– Location: Light/No Light– Atmosphere: Aerobic/Anaerobic

NETRA

Controlling Microbiological Growth

Water Quality– Eliminate organic contaminants (food)– No food = No bugs

• Bugs are carniverous – A forest feeds itself

System Design Considerations– Clean tower and sumps, cover decks

Chemical Treatment with Biocides

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Chemical Treatment With Biocides

• Oxidizing Biocides

• Non-oxidizing Biocides

• Biodispersants

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Losses in Pipelines due to Friction Head & consequent Energy Losses

119

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120

Pump designed for 630 m3/hr was delivering 450 m3/hr due toSevere fouling. Corrosion products

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121

NETRA

IMPURITIES IN WATER

• BACTERIA & VIRUSES

• MICRO – ORGANISMS

• TURBIDITY

• COLOUR

• MINERALIZATION

• METALLIC

• DISSOLVED GASES

• AMMONIA

• ORGANIC MATTER

• POLLUTANTS

122

NETRA

ORGANIC IMPURITIES IN WATERS

Insoluble Organic Matter:

• Debris of vegetable and animal origin• Micro-organisms (living & dead)• Oily material• Humic matter

Soluble Organic Matter: • Humic matter• Fatty acids• Nitrogenous matter (proteins, peptides, & Amino acids)• Saccharides & Sugars• Dissolved Organic gases (such as Methane)• Soluble extracts of Vegetable and animal matter• Synthetic Organic Compounds

123

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MAKE - UP WATER TREATMENTS

• ORGANIC MATTER REMOVAL

• CLARIFICATION - ALUM, PEs, PAC

• FILTERATION

• ULTRAFILTERATION

• NANOFILTERATION

• REVERSE OSMOSIS

• ACTIVATED CHARCOAL

• SOFTENING - LIME, ION EXCHANGE

• BIOCIDE ADDITION

• DEMINERALIZATION

• OXYGEN REMOVAL

• BOILER WATER TREATMENT

124

NETRA

Problems associated with Organic Matter, Microbiology in CW Systems

1. Improper Pre-treatment of water (both for DM application & CW system)

2. Foaming in the waters (Loss of efficiency)

3. Biofouling & Microbiologically Induced Corrosion of Condenser tubes, Condenser tube leakages (consequent higher boiler tube corrosion & loss of efficiency)

4. Fouling/Microbiologically Induced corrosion of pipelines (Improper flow of water/pipe leakage, Non-availability of fire fighting systems)

5. Fouling and reduced efficiency of fills of cooling towers (special cleaning or fill replacements)

6. Fouling/Corrosion of Ducts/Pumps (Efficiency loss)125

NETRA

PROBLEMS DUE TO LEAKAGEFROM CONDENSER TUBES

Many of the corrosion problems in fossil fuel boilers, LP steam turbines and feedwater heaters have been traced to leaking condensers.

Tube leaks allow the ingress of cooling water into the steam-water cycle.

The very nature of the condenser tends to increase a problem with cooling water leakage, in that the condensate side of the condenser operates in a vacuum and thus any leak in a tube wall or other connection will allow cooling water to be drawn into, and contaminate, the pure condensate.

The acceptable impurity levels in cooling water are much higher than those acceptable for the condensate.

This can be a problem in both once-through systems and in recirculating systems.

126

NETRA


For example, in the event of a leaking tube, cooling tower water can present a contamination problem nearly as bad as seawater because of its high hardness and high concentrations of other dissolved solids.

Cooling water also often contains chemicals added to control biofouling, scale and silt. Condenser corrosion problems have increased in the past few years, in part, as a result of higher pollution in cooling water.

Use of treatment plant effluent as tower makeup is growing and also increases the risk of problems.

Condensate polishers can provide some protection against impurity ingress to the cycle; however, their capability can be overwhelmed by condenser leaks and, during larger leaks, can be exhausted within minutes.

The effect of condenser tube leaks and damage are manifest. These include effects on cycle chemistry, steam generators and boilers, the turbine, and plant discharges.

127

NETRA

EFFECTS OF CONDENSER TUBE LEAKS AND DAMAGE

The ingress of cooling water into the steamside of the condenser will result in its subsequent distribution into the water-steam cycle.

Carried throughout the cycle, there is a significant potential for damage to the steam generator or boiler, and turbine.

Steam generators and boilers are typically the components most susceptible to damage by contaminants from condenser leakage.

In some cases degradation of steam generator materials has progressed to the point that replacement of the steam generators has been required.

A number of studies have demonstrated the incentive to minimize aggressive impurities introduced into the steam generator feedwater by leakage of cooling water and/or air into the condenser.

Boiler tube failure mechanisms in fossil-fueled power plants that are influenced by condenser tube leaks include: hydrogen damage, corrosion fatigue, pitting and stress corrosion cracking.

128

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129

• Corrosion due to water flowing inside the pipe results in development of rough surfaces and additionally the corrosion products reduce the effective internal diameter of the pipe. • In many small diameter pipes the nodular growth (tuberculation) causes reduction of diameter of pipe to almost 1/10th or lesser of the original diameter besides creating highly rough surface.

• This results in very high-energy losses and non-availability of sufficient water required for specific purpose such as cooling or drinking.

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130

Hazen Williams Equation:

Q = 0.278 CS 0.54 D 2.63 Q in m3/s, D in m, S = H/L

V = 1.318 C R h 0.63 S 0.54 (USC Units)

V = 0.849 C R h 0.63 S 0.54 (SI Units)

Where, Rh is the hydraulic radius, S is the head loss per unit length (hf/L), and C is a

roughness coefficient associated with the pipe materialFriction loss based on the Hazen-Williams formula is:

f = 0.2083 x (100/C)1.852 x (Q 1.852 /d i 4.8655).

In this formula, the following apply:f = friction head loss in feet of water per 100 feet of pipe;C = constant for inside pipe roughness;Q = flow in US gallons per minute (gpm);d i = inside diameter of pipe in inches

(1 US GPM = 0.227 m3/hr)

NETRA


In this system, the full-stream filter protects the cooling tower and other process equipment in the loop.

131

NETRA

Scaling in Condenser tubes

132

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Biofouling in Condenser tubes

133

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Corrosion in CW System

134

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Corrosion Control in CW System

135

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Fouling of PVC Film Type Fills of CT

136Loss of around Rs. 1.2 Crore per 500 MW unit per year

NETRA

Chemical cleaning of severely scaled condensers

137

Heat Rate Improvement = 183 Kcal/kwh

Annual Gain = Rs. 15 Crores

Full Load operation against 80% before cleaning

NETRA

Other Problems of CW System

138

Eutrefication of reservoir RCC corrosion of Cooling tower structure

Severe foaming at CW Pump and in CW Foreway due to organic contamination

NETRA

Corrosion, Fouling, Biofouling & Deposition in CW systems

• Corrosion, scaling, biofouling in CW system - water chemistry & microbiological species

• Fouling of CT Fills – Suspended matter and microbiological species (No treatment)

• Severe scaling, under-deposit corrosion – Poor water chemistry control. biofouling

Recommendations

• Development & execution of site specific chemical cleaning to restore heat transfer of condensers & CT fills.

• Development of site specific chemical treatment program for controlling scaling, fouling & corrosion in CW systems & CT Fills.

• Installation of side-stream filters

• Dosing of chlorine dioxide with chlorine as biocide to control microbiology.

139

NETRA

Corrosion in Condenser & heat exchanger tubes

• Condenser tubes (Brass) - stress corrosion cracking. Dezincification & under deposit corrosion due to water chemistry/microbiological species

• Corrosion of Condenser Water Boxes: Tube plates – Poor quality of cooling water & poor corrosion protection

• Aluminum Brass tube failures by seawater – Pitting, deposition, MIC

• Failure in transit/storage – Crevice corrosion, pitting, thinning due to ingress of seawater

Recommendations

• Replacement of brass tubes with copper- nickel tubes for fresh water service.

• Replacement of brass tubes with titanium tubes for seawater service.

• Application of anticorrosive coatings and cathodic protection system for water boxes/tube plates

• Replacement of damaged SS tubes with new tubes.

140

NETRA

Corrosion in pipelines carrying water (internal side & external side)

• Internal side - water chemistry & microbiological species, stagnancy

• External Side – Soil, stray current

Recommendations

• Design & execution of cathodic protection systems.

• Replacement pipe material with HDPE, SS

• Anticorrosive coatings

• Dosing of chlorine dioxide with chlorine as biocide to control microbiology.

• Water treatment

• Ensuring regular flushing of pipelines

141

NETRA

Improvements in energy efficiency and Corrosion Control of pumps

142

NETRA

Some Cases of effect of Organics & Microbiology

143

Badarpur – Problem of High Conductivity of DM Water, Condenser tube leakage,Problem of control of boiler tube, tripping of SPU – reason Untreated sewage

Singrauli – Biofouling in condenser tubes, microbiologically induced corrosion (MIC)

Tanda – Severe biofouling, scaling & MIC in condenser tubes (loss of generation)

Kahalgaon - Severe biofouling, scaling & MIC in condenser tubes (loss of generation),Problem of non-availability of sufficient DM water, Fouling of PVC fills (stage II)

Faridabad – Fouling of PVC fills, Eutrefication of make up water reservoir, pH variation

Kawas – Severe foaming in CW system, (Casuarina tree leaves)

Kayamkulam – Foaming in CW system, Yellow colour in water, choking of NOx filtersSevere biofouling in raw, CW pipelines, HVAC lines, DM feed lines

Talcher Kaniha, Rihand, Dadri, Gandhar, Auraiya, Faridabad, Unchahar, - Fill fouling

Rihand, Vindhyachal, Talcher Kaniha – MIC in clarified water pipelines

NETRA

Problems due to Organic Matter, Microbiolgy Case 1

HIGH ORGANIC MATTER LOADING (UNTREATED SEWAGE) IN RAW WATER:

• Acute problem of obtaining required quality of DM water throughout the year • Problem more severe in summers

• Due to high untreated sewage – proper clarification is very difficult

• Chlorine demand is very high (100 – 200 ppm)

• DM water conductivity goes as high as 0.8 us/cm (4 us/cm in DM tanks)

• pH of Boiler water drops to 7.5 – requires caustic dosing

• High Conductivity water causes deviations in Boiler & Stator water Chemistry

REMEDIAL MEASURES:

• Aeration of the make up water to remove BOD/COD

• Installation of Sewage Treatment Plant prior to clarifiers & Chlorine Dioxide

• Installation of Ultrafilteration system/RO water system prior to DM Plant

• Alternate source of raw water – borewell for DM plant

144

NETRA

Effects of organic matter on CW Systems – Case 2

145

Muttom Pump house

Balancing Reservoir

(175000 KL)

Raw waterPumps

StillingChamber

Clarified waterStorage tank

(600 KL) Gravity sandFilters (3 beds)

Filtered waterStorage tank

(600 KL)

Filtered waterReservoir

SAC DG SBA MB

DM WaterStorage tank

Clf 1

Clf 2

ACF

Feed to NOxControl Filters

DM FeedHVAC Make upService Water

Potable water(Plant & Township)

Cl2 Lime

8 KM MS 500 mm Pipe

Pumping once in a week for 3 days when units operating

Good filterability of water

CT Make-up (85%)

Movement of water between 1.9 – 2.3 m(Total 2.3 m depth) Acacia leaves/seedsAt the bottom, poor filterability

Daily basis8 – 10 hours

Continuous5.5 NTU Problem in clarification

No proper coagulation2.8 – 3.0 NTU

Problem in Ion exchange

Choking of NOx filters(4 filters replaced in 1 – 3 days, cost of Each filter = Rs. 1400/-) (Earlier replacementIn 15 – 18 days)

Schematic of water intake & usage at Kayamkulam- Problems due to falling Acacia leaves & seeds

NETRA

Effects of organic matter on CW Systems – Case 3

146

NETRA

Corrosion in RCC Structures of Cooling Towers

• Corrosion induced damages to RCC Structures of Cooling Towers & other structures– More severe in coastal regions (Simhadri, Dabhol)

Recommendations • Repair and rehabilitation of RCC

structures along with application of cathodic protection system

• Design of cathodic protection system for RCC structures such as natural draft cooling towers, Ash handling, ESP, etc has been prepared & sent to site. This is to be integrated with civil repair job

147

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Condition Assessment Criteria

S.No. Concrete Design Strength (N/mm2)

Potential (mV Vs Cu/CuS04)

Corrosion Condition

Electrical Resistivity (KiloOhm cm)

Corrosion Condition

1 M - 30 > - 200 Low > 20 Negligible

2 M - 30 - 350 to - 200 Intermediate 10 to 20 Low

3 M - 30 < - 350 High 5 to 10 High

4 M - 30 < - 500 Severe < 5 Very High

Condition Assessment Criteria

NETRA

22 30 27

15 30 28

30 28 17

ESP Buffer Hopper

Rebound Hammer: (N/mm2)

-457 -444 -408

-455 -434 -411

-426 -436 -423

Half cell potential :(mV)

3.6

1.9 4.4 1.9

2.8

Electrical Resistivity :( kilo ohm cm)

Condition Assessment data

High Corrosion

Very High Corrosion

NETRA

46 52

48 48

47 46

Ash Slurry Sump

Rebound Hammer: (N/mm2)

-382 -402 -417

-425 -395 -435

-410 -457 -415

Half cell potential :(mV)

3.1 2.8

2.1 2.4 2.3

1.8

2.1



High Corrosion

Very High Corrosion

NETRA

Bottom Ash Hopper

Rebound Hammer: (N/mm2) Half cell potential :(mV)


40 45 35

45 26 35

38 25 48

-557 -441 -409

-522 -498 -432

-514 -458 -465

7.9 7.9

4.5 6.2 7.3

9.4 7.0


High to Very High Corrosion

High Corrosion

NETRA

Cooling Tower

Rebound Hammer: (N/mm2) Half cell potential :(mV)


20 23 28

21 21 20

20 25 22

-357 -384 -391

-334 -352 -380

-410 -403 -415

1.2 2.8

2.1 2.4 2.3

1.8 2.1


High Corrosion

Very High Corrosion

NETRA

Samples from inner surface of NDCTs

NETRA

Sampling Locations on NDTCs

NETRA

Chloride PenetrationThe analysis work is in progress, but the initial results of the tests of about 20 samples showed the chloride content ranges from 0.10 to 0.80% by weight of cement, indicating the chloride content is more than critical in most of these samples.

pH ValueThe laboratory analysis of the samples is in progress. However, the results of the pH test conducted on about 20 samples so far revealed the pH value ranges from 8.0 to 12.5, indicating chemical attack in some of the samples.

Preliminary results from CBRI

ACI 222R - 6

NETRA


Rebound Hammer TestRebound hammer tests were conducted at about 60 locations to assess the quality of concrete using its surface hardness property. The detailed analysis of the results in progress, but the results indicate that the mean concrete strength on these locations range between 20 and 40 MPa. It appears that at some locations the concrete strength is lower than that of the specified grade of concrete i.e. M30.

Half Cell Potential MeasurementThe results have been obtained for Half Cell Potential tests conducted at about 450 points. The detailed interpretation alongwith other test results is in progress. However, the results show that the Half Cell Potential of the steel reinforcement varies from -165 to -550 mV representing active corrosion at several locations. According to ASTM C876 potentials over an area more negative than −200 mV (Copper Sulphate Electrode) indicate probability of active corrosion.

Concrete ResistivityThe results of the concrete resistivity tests varied between 0.4 to 26.0 kΩ.cm. The resistivity greater than 20.0 kΩ.cm shows low corrosion rate. However, lower values the resistivity value indicate higher corrosion rates. In most of the locations lower values of resistivity have been recorded, means there are corrosion taking place.It may please be noted that the results of the above mentioned tests are influenced by various parameters and must be used carefully. The detailed analysis along with these parameters will be covered in the subsequent reports when all the test results are obtained.

NETRA


Concrete Core TestingTo understand the status of the quality of concrete inside the cooling tower shells by visual observations, compressive strength testing, pH, Carbonation, chloride penetration tests 44 nos. of concrete cores (22 nos. from each cooling tower) and 60 mm diameter were extracted. The locations were selected keeping in view the suspected corrosion and easy & safe accessibility. The finishing work for compressive test of these cores is in progress. In few of these cores internal damages/cracks, inhomogeneity and honeycombing have been noticed.

CarbonationThe results of carbonation test have revealed that in most of these core carbonation has taken place on the surface only and it is not the main cause of concrete deterioration.

NETRACondition Assessment of RCC Structures

NETRAESP Buffer Hopper Structures

NETRABottom Ash Structures

NETRAPump House Structures

NETRA

Table 2 – EPRI Guidelines for Cooling waters

Water Quality EPRI Guidelines Remarks

Parameter Units

Ca Mg/l CaCO3 900 (max)

Ca X SO4 (Mg/l)2 500000

M Alkalinity Mg/l CaCO3 30 – 50220 - 250

Without Scale InhibitorWith Scale Inhibitor

Mg X SiO2 Mg/l CaCO3 X

Mg/l SiO2

35000

SO4 Mg/l

SiO2 Mg/l 150

PO4 Mg/l

Fe (Total) Mg/l < 0.5

Mn Mg/l < 0.5 166

NETRA


Water Quality EPRI Guidelines Remarks

Parameter Units

Ca Mg/l CaCO3 900 (max)

Ca X SO4 (Mg/l)2 500000

M Alkalinity Mg/l CaCO3 30 – 50220 - 250

Without Scale InhibitorWith Scale Inhibitor

Mg X SiO2 Mg/l CaCO3 X

Mg/l SiO2

35000

SO4 Mg/l

SiO2 Mg/l 150

PO4 Mg/l

Fe (Total) Mg/l < 0.5

Mn Mg/l < 0.5 167

NETRA


Cu Mg/l < 0.1

Al Mg/l < 1

S Mg/l 5

NH3 Mg/l < 2 For copper based alloys present in the system

pH 6.0 – 7.27.8 – 8.4

Without Scale InhibitorWith Scale Inhibitor(Higher operating pH is possible with new alkaline treatments)

TDS Mg/l 70000

TSS Mg/l < 100< 300

For Film type FillSplash type Fill

BOD Mg/l

168

NETRA


COD Mg/l

LSI < 0

RSI > 6

PSI > 6

169

NETRA

170

NETRA

Methods of removal of organic matter

Ion Exchange methods.

Membrane based filteration methods.

Aeration

Oxidizing biocides – Ozone, Hydrogen peroxide, Chlorine, Chlorine dioxide, Bromine, etc.

Non-oxidizing biocides – Glutraldehyde, Quaternary. Ammonium salts,

Application of anticorrosive coating based on vinyl ester glass flake or 100% solids epoxy on the condenser water box/inlet & outlet of CW pipes.

Recommendations for change in chemical treatment program based on site specific requirements.

Aerobic Sewage treatments

Anaerobic Sewage treatments

Activated charcoal

Physical cleaning of tanks/reservoirs, Exposure to steam/high temperature

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Current practices on Remedial and Control measures adopted

Development of site specific chemical and mechanical (high pressure jet, bullet cleaning, etc) cleaning procedure (including supervision) to remove scale deposits from condensers, heat exchangers, and pipelines.

Development of non-proprietary chemical treatment programs for CW systems to control scaling, fouling, biofouling and corrosion.

Change of condenser tube material from brass to copper-nickel 90/10 in fresh water and to Titanium grade II for seawater applications

Change of condenser tube material in air cooling zone to copper-nickel 70/30 in place of copper-nickel 90/10 to reduce the ammonia grooving attack.

Design and installation of sacrificial cathodic protection system based on Magnesium based alloy.

Application of anticorrosive coating based on vinyl ester glass flake or 100% solids epoxy on the condenser water box/inlet & outlet of CW pipes.

Recommendations for change in chemical treatment program based on site specific requirements.

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Use of chlorine dioxide as a biocide in addition to continuous chlorine dosing for better microbiological control.

Application of online condenser tube cleaning systems to control biofouling in the condensers.

Development of chemical cleaning/treatment program for PVC film type cooling tower fills to control biofouling/fouling.

Application of anticorrosive coatings for CW system

Application of online conductivity, ATP, corrosion, fouling and biofouling monitors for continuous monitoring and treatment optimization.

Change of raw water pipes to HDPE

Recycling of CW blow down water after treatment for use as make up to cooling water.

Installation of advanced sewage treatment plants/aeration plants for treating organically contaminated water as input to CW system.

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Trimming of trees causing organic fouling in the CW system and cleaning of reservoirs to remove organic foulants from the bottom of the reservoirs.

Prevention of ingress of surface water/storm water in the CW system.

Assessment of structural damage due to RCC corrosion and application of cathodic protection of RCC in cooling tower structures.

Prevention of ingress of fly ash in CW system.

Applications of alternate chlorination methods such as hypochlorite, elecrochlorination, etc wherever gas chlorine dosing systems are not available.

Use of Polyaluminum chloride in place of/in addition to alum as flocculating aids for better clarification and lesser corrosion.

Application of side stream filters and make up water filters to remove suspended matters 174

NETRA

Missing links and New Requirements for controlling problems of CW systems

CW system is the most neglected area of the power plants and there is lack of proper information to plant operators regarding CW system problems.

There is no systematic assessment of cooling water characteristics and the control measures such as material selection, anticorrosive coatings to be applied, cathodic protection to be applied, suitable chemical treatment program, filters installation, etc. to be adopted at design stage.

Chemical treatment is still a mystery to most power station chemists/operators.

No systematic corrosion audit is carried out to assess the efficacy of the control measures adopted.

Concept of online monitoring in CW systems is still a distant dream at power plants.

Cost of control measures is taken as expenditure rather than as investment for improving plant availability, reliability and efficiency.

Lack of proper specifications for operation and monitoring chemical treatment programs due to lack of knowledge on the treatment program and the chemicals used.

175

NETRA

Missing links and New Requirements for controlling problems of CW systems

Lack of suppliers for online monitors making it difficult for procurement. Lack of condenser seepage detection technologies

Apprehensions on use of alternate biocides

High costs of treatment programs

Lack of dedicated manpower for attending CW problems

176

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STATIONS FOR WHICH CW SYSTEM RELATED STUDIES CONDUCTED

NTPC Stations• Badarpur• Faridabad• Dadri – Gas & Thermal• Auraiya• Anta• Kahalgaon• Tanda• Vindhyachal• Rihand• Singrauli• Talcher Kaniha• Unchahar• Kawas• Gandhar• Kayamkullam• Talcher Thermal• Ramagundam• Simhadri

Other than NTPC Stations• Bathinda - Punjab• Ropar - Punjab• Kota Thermal - Rajasthan• Panipat - Haryana• Ennore – Tamil Nadu• Suratgarh – Rajasthan*• Lehra Mohabat – Punjab• Muzzaffarpur - Bihar• Bhillai – NTPC - SAIL• IP power Station - Delhi• Ratanagiri – RGPPL (Dabhol)• Rourkela – NTPC – SAIL• Jindal Power Limited – Raipur• Faridabad Thermal • Neyveilli • Puducheri Power Corp. limited

* Work to start

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Online biofouling Monitoring

NETRA

ONLINE BIOFOULING MONITORING

NETRA

ONLINE MONITORING SYSTEMS

NETRA

ONLINE CORROSION/FOULING MONITORING

NETRA

BIOFILM ACTIVITY MONITOR

Biofouling Monitoring – Differential Pressure

NETRACORROSION RATE MONITORS FOR FIELD

ONLINE CORROSION MONITORING SYSTEMS

NETRA

184

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Oxidizing biocides & Microbes

185

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R&D CENTRE

COMMONLY USED OXIDISING COMMONLY USED OXIDISING BIOCIDESBIOCIDES

APPLICATION METHODS

ACTIVE SPECIES

DOSAGE/DURATION

COST EFFECTIVENESS

CHLORINE BASED

BROMINE BASED

CHLORINE DIOXIDE

FED AS A GAS OR AN AQUEOUS SOLUTION

HYPOCHLOROUS ACID HYPOCHLORITE ION

USUALLY 0.2mg/L RESIDUAL CHLORINE FOR 2 HOURS/DAY (4 TIMES FOR 30min. EACH)

USUALLY MOST COST EFFECTIVE

FED AS AQUEOUS SOLUTION OR GENERATED VIA OXIDANT REACTION

HYPOBROMOUS ACID HYPOBROMITE ION

USUALLY 0.1mg/L RESIDUAL BROMINE FOR 2 HOURS/DAY(4 TIMES FOR 30min.EACH)

GENERALLY 50-100% MORE COSTLY THAN CHLORINE

GENERATED AT SITE AND MIXED WITH WATER

CHLORINE DIOXIDE (ClO2)

A RESIDUAL 0.05-0.1 mg/L FOR 1 HOUR/DAY(4 TIMES FOR 15 min. EACH)

GENERALLY 700-800% MORE COSTLY THAN CHLORINE

186

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R&D CENTRE

EFFECTIVENESS OF CHLORINE AS A BIOCIDE

EFFICIENCY OF CHLORINE AT DIFFERENT pH IN COOLING WATER SYSTEM

4 5 6 7 8 9 10 11

80

60

40

20

0

20

40

60

80

100

O°C

HOBr

Per

cen

t io

niz

ed f

rom

(O

Cl -

or

OB

r- )

Per

cen

t u

n-i

oniz

ed f

orm

(H

OC

I - o

r H

OB

r- )20°C

HOCI

pH

Cl2 + H2O OHCl + H+ + Cl–

(Hypocblorous acid)

HOCl + OH– H+ + ClO–

(Hypocblorite ion)

HOCl + OH– H2O + ClO–

187

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R&D CENTRE

EFFECT OF pH ON THE EFFECT OF pH ON THE DISSOCIATION OF HYPOCHLOROUS DISSOCIATION OF HYPOCHLOROUS

ACIDACID

pH

4

5

6

7

8

9

HClO

100

99.7

96.8

75.2

20.0

Negligible

188

NETRA

EFFECTIVENESS OF CHLORINE DIOXIDE AS A BIOCIDE

The biocidal efficiency, the stability, and the effects of pH on the efficiency of some disinfectants

Disinfectant Biocidal Efficiency*

Stability* Effect of the pH Efficiency (pH = 6-9)

Ozone 1 4 little influence

Chlorine Dioxide

2 2

efficiency slightly increases with the increase of pH

Chlorine 3 3 efficiency decreases considerably with the increase of pH

Chloramines 4 1 little influence

189

NETRA

ClO2 and Cl2 effectiveness in seawater at pH 8,2

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120 150 180 210 240 270 300

Contact time second

Org

anis

m s

urv

ival

%

Cl2 1,0 mg/l

ClO2 0,3 mg/l

Cl2 3,0 mg/l

ClO2 1 mg/l

- Cl2 demand = 1,7 mg/l- ClO2 demand = 0,9 mg/l

Chlorine dioxide and chlorine comparison on bacteria

190

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0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10

0

10

20

30

40

50

60

70

80

90

100

ch

lori

ne

sp

ec

ies

in s

olu

tio

n %

pH

2ClHOCl

OCl-

CHLORINE SPECIES FORMATION IN WATER ACCORDING TO pH

191

NETRA

Comparison of Disinfectants

192

Chlorine Chlorine Dioxide

Ozone UV

residual for many hours

residual for several days

residual for minutes

no residual

chlorine gas, hypochlorite or electrolysis

Chemicals: HCl or H2SO4

and NaClO2

dry ambient air, electric energy

electric energy

disinfection capacity: medium

disinfection capacity: strong

disinfection capacity: strong

disinfection capacity: medium

dependence on pH: extreme

dependence on pH: none

dependence on pH: less

dependence on pH: none

DBP’s: THM’s, chloramines, chlorophenols and other AOX (limit 0.03 ppm)

DBP’s: chlorite (limit 0.8 ppm)

DBP’s: bromate (limit 0.01 ppm)

DBP’s: nitrite (limit 0.5 ppm)

NETRA

DISADVANTAGES OF CHLORINE

• Chlorine reacts with organics and hence exerts a Chlorine Demand and hence additional Chlorine has to be dosed to overcome the demand before a free residual is available to act as biocide.

• Efficacy of Chlorine is pH dependent. The Hypochlorous acid, which is a desired component, will dissociate to H+ and Ocl- ions with increase in pH. HOCl content will decrease from 80% at a pH of 7 to 20 % at a pH of 8.

• Chlorine is highly corrosive and exposure to it can lead to rapid corrosion of unprotected metal.

• Chlorine is a toxic gas and hence storing and handling Chlorine poses severe risk to Environment, health and safety.

• Reaction of Chlorine with organics produce Trihalomethanes or THM’s which are known to be carcinogenic and hence many European countries and America are bringing use of Chlorine under increased regulation.

• Chlorine does not have the ability to penetrate and remove biofilms formed.

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MAJOR ADVANTAGES OF CHLORINE DIOXIDE • Chlorine dioxide is a very powerful oxidizing biocide.

• Chlorine dioxide is effective over a broad pH range.

• Chlorine dioxide has 2.5 times the oxidizing capability of Chlorine.

• Chlorine dioxide is a selective oxidant.

• Unlike Chlorine, Chlorine dioxide remains a true gas dissolved in solution. Since Chlorine dioxide does not react with water, it retains its biocidal effectiveness even over a wide pH range.

• In cooling water treatment applications, where various levels of contaminants are present, consumption of Chlorine is significantly higher as that compared to Chlorine dioxide to maintain similar residual levels.

• Required contact time for Chlorine dioxide is lower compared to Chlorine.

194

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MAJOR ADVANTAGES OF CHLORINE DIOXIDE

• Chlorine dioxide destroys phenols.

• Chlorine dioxide has the ability to penetrate and remove biofilm formed and kills bacteria, spores and viruses.

• Chlorine dioxide is less corrosive than chlorine and does not hydrolyze to form acid.

• Chlorine dioxide does not form chloramines.

• Chlorine dioxide does not form trihalomethanes upon reaction with organic matter.

195

NETRA

CONDENSER TUBE DAMAGEMECHANISMS

Damage Mechanism Most Commonly Affected

Material(s)

Tube Impact Damage from

Tube Vibration

All materials potentially susceptible; thin

Walled titanium tubes have been more

susceptible if Support spacing is inadequate.

Fatigue & Fatigue

Corrosion from Tube

Vibration

All materials potentially susceptible; thin

Walled titanium tubes have been more

susceptible if Support spacing is inadequate.

Screening Table for Condenser Tube Steam-side Damage Mechanisms

196

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Summary of Primary and Secondary Targets for Drum-type Boilers under Steady State Operation (expressed as ug/kg unless otherwise stated)

Boiler Class

Parameter 60 Bar –Gas Fired

100 Bar –Coal Fired



Feed-water

Conductivity(u/S/cm at 25°C)

<-local decision to achieve primary target in boiler water->

Sodium (Na) <-local decision to achieve primary target in boiler water->

Sulphate (SO4) <-local decision to achieve primary target in boiler water->

Dissolved Oxygen-In condensate

<50 <50 <50 <50

Dissolved Oxygen-In Feed

<7 <7 <5 <5

Oil <200 <200 <200 <200

TOC

Hydrazine (N2H4) <— 2 x Dissolved Oxygen concentration —>

197

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Ammonia (NH3) <— < 500 when copper alloys present —>< 1000 when copper alloy absenti.e. when SS or Titanium are present

pH (at 25oC) <— 8.8 - 9.2 (or 8.8 - 9.4 when copper —>alloys absent)

Total Metals <20 <20 <20 <20

Spray Water Sodium (Na)

10

(without RH)

10(without RH)

5(without RH)

5(without RH)

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Boiler-water

1. Non-volatile Phosphate Treatment

Chloride (NaCl) as Chloride

< 3000 < 2000 < 1000 < 500

Silica (SiO2)(at pH - 9)

< 5000 < 1500 < 300 < 200

Sulphate (SO4) <-local decision to achieve primary target in boiler water->

Disodium/ TrisodiumPhosphate

2000To

6000

2000To

4000

1000To

2000

1000To

2000

All Volatile Akali Treatment

Chloride (NaCl) as Chloride

NA < 120 < 120 NA

Silica (SiO2)(at pH - 9)

< 350 <250 < 150 < 100

Sulphate (SO4) NA LD LD NA

199

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Saturated Steam

Silica (Sio2) <20 <20 <20 <20<10 (depending upon

design) with RH

Sodium (Na) < 20 < 10 < 6 < 5< 3 (On AVT

200

NETRA


The demands placed on the condensers of utility generating units are significant. Functionally, a condenser must condense several million pounds/kilograms per hour of wet (up to ~ 15% moisture) steam at low temperatures (~ 38°C (100°F)) while producing low absolute pressures (~ 6.8 kPa (1 psia)). It must degasify condensate to the ppb level. These tasks must be done while also:

• Serving as an impervious barrier between steam/condensate and circulating water.

• Permitting only limited air in-leakage.

• Contributing minimal corrosion products to the condensate in a “hostile” environment that is aerated, wet and at high velocity.

Despite the significant demands placed on the condenser and exacting penalties for condenser leaks, the condenser often does not get the attention it deserves.

201

NETRA

PREVENTION OF INGRESS OF COOLING WATER IN FEED CYCLE

• Detailed failure analysis of condenser tubes to assess the cause of failure.• Checking of Baffles & proper control of boiler water chemistry to prevent steam side corrosion.• Development & execution of suitable site specific chemical treatment to control corrosion, fouling, scaling & biofouling/MIC including biocides.• Development & execution of cleaning methodology to remove deposits, foulants, corrosion products to restore heat transfer/under deposit corrosion.• Design & Installation of Cathodic Protection systems for condenser water boxes and tube plates.• Recommendations of anti-corrosive coatings for condenser water boxes, tube plates, CW ducts, etc.• Recommendations for online monitoring equipment to assess the corrosive, fouling, biofouling tendencies of cooling waters.• Efforts are being made to develop suitable sensors/probes for measurement of corrosion of boiler tubes on real time basis.

202

NETRA

Problems

Cemical Treatments Corrosion Scaling Fouling Microbes

Chromates X

Zinc X

Molybdates X

Silicates X

Polyphosphates X X

Polyol esters X X

Phosphonates X X

Natural Organics X X

Polyacrylates X X

Non Oxidizing biocides X X

Chlorine X

Ozone X

CW Treatment Chemicals

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Treatment Chemicals Concentrations

Chromate/Zinc 5-10 ppm CrO4/5-10 ppm Zn

Chromate/Phosphate/Zinc 10-30 ppm CrO4/3-5 ppm PO4/3-5 ppm Zinc

Chromate/Polysilicates 5-10 ppm CrO4/5-10 ppm SiO2

Chromate/Molybdate 10-30 ppm CrO4/1-5 ppm MoO4

Chromate/Phosphonate 5-10 ppm CrO4/3-5 ppm Phosphonate

Corrosion Inhibition

204

NETRA

Treatment Chemicals Concentrations

Polyphosphate 5-10 ppm PO4

Ortho/Poly Phosphate 10-30 ppm Total PO4

Polyphosphate/Zinc 10-20 ppm PO4/1-3 ppm Zn

Zinc/Phosphonate 3-5 ppm Zn/3-5 ppm Phosphonate

Zinc/Tannin/Lignin 3-5 ppm Zn/50-100 ppm Tannin + Lignin

Polysilicate 10-15 ppm SiO2

Molybdate/Phosphonate 5-20 ppm MoO4/3-5 ppm Phosphonate

Polysilicate/Molybdate 10-20 ppm SiO2/1-3 ppm MoO4

Phosphonate/Polyacrylate 5-20 ppm Phosphonate/10-20 ppm Acrylate


205

NETRA

Inhibitor Metal Limitations

Steel Copper Aluminum Ca ppm pH TDS ppm

Chromate Excell Excell Excell 0-1200 5.5-10.0 0-20000

Polyphosphate Excell Attacks Attacks 100-600 5.5-7.5 0-20000

Zinc Good None None 0-1200 6.5-7.0 0-5000

Polysilicate Excell Excell Excell 0-1200 7.5-10.0 0-5000

Molybdate Good Fair Fair 0-1200 7.5-10.0 0-5000

Copper inhib. Fair Excell Good 0-1200 6.0-10.0 0-20000


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Prediction of Water Characteristics by the Langelier Index

LSI Tendency of Water

+2-0 Scale-forming and for practical purposes

noncorrosive

+0.5 Slightly scaling and noncorrosive

0.0 Balanced but pitting corrosion possible

-0.5 Slightly corrosive and nonscale-forming

-2.0 Highly corrosive

207

NETRA

Prediction of Water Characteristics by the Ryznar Index

Ryznar Stability Index Tendency of Water

4.0 - 5.0 Heavy Scale

5.0 - 6.0 Light Scale

6.0 - 7.0 Little Scale or Corrosion

7.0 - 7.5 Corrosion Significant

7.5 - 9.0 Heavy Corrosion

9.0 and Higher Corrosion Intolerable

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Material(s)

Erosion-Corrosion Copper alloys

Sulphide Attack Copper alloys

Pitting Some Stainless Steels, Copper alloys

Crevice Corrosion Copper alloys, some Stainless Steels

Dealloying Copper alloys

Microbiologically Induced Corrosion

Copper alloys, Stainless Steels

Galvanic Corrosion Requires susceptible combination of metals

Waterside SCC Copper alloys, 300 Series Stainless Steels

Screening Table for Condenser Tube Water-side Damage Mechanisms

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Material(s)

Hydriding Titanium

Hydrogen Embrittlement Ferritic Stainless steels

Cleaning Damage Copper alloys most affected by mechanical Cleaning damage.

Copper alloys & Stainless steels most affected by chemical cleaning damage

Cavitation Damage from Tube Vibration

All materials potentially susceptible

Screening Table for Condenser Tube Water-side Damage Mechanisms

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Material(s)

Steam side Erosion

(Impingement damage)

Copper alloys are most affected but SS and Titanium can also be affected

Impact damage Any Tube Material is susceptible

Condensate corrosion (Ammonia Grooving)

Copper alloys

Steam side SCC Copper alloys

Fretting and Fretting Corrosion from Tube Vibration

All materials potentially susceptible, although

thin-walled titanium tubes have been more susceptible if tube support spacing is inadequate

Screening Table for Condenser Tube Steam-side Damage Mechanisms

211

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OVERVIEW

1. Issues involved in Management of CW systems

2. Activities of Corrosion Analysis, Monitoring & Control Laboratory

3. Cost of Corrosion in Electric Industry

4. Water requirements at Power plants

5. CW Systems and impact of water on plant components

6. RCC Corrosion & Condition Assessment of Structures

7. Case studies on Organic/Microbiological Associated Problems in CW Systems

8. Recovery of water from Flue gas & applications of condensed acid

9. Conclusions

212

water chemistry workshopaksinha

Documents

netra corrosion corrosion

corrosion of structures

ohnetra corrosion

corrosion products

power plants corrosion

corrosionrelated problems

underdeposit corrosion

total corrosion cost